In a communications network, it is often desirable to identify and distinguish one transmitter from other transmitters operating within the network. For example, in the radio telephone industry, a cellular telephone system utilizes an electronic serial number (ESN) and a mobile telephone identification number (MIN) to provide a unique identification for each transmitter. When an individual subscriber or other authorized user of a particular cellular telephone wishes to place a phone call, he dials in a telephone number and presses the "Send" button. In response, his cellular telephone transmits its ESN and MIN to the cellular network so that the individual subscriber can be charged for the telephone call.
Unfortunately, unscrupulous individuals illegally operate cellular telephones by counterfeiting the ESN and MIN of a valid subscriber's telephone in order to obtain illegal access to the cellular network without paying for the service. The ESN and MIN of a cellular telephone can be obtained by a counterfeiter electronically monitoring the initial transmission of the user's telephone, and then programming the detected ESN and MIN into another telephone for illegal use. Thus, the mere transmission of the authentic ESN and MIN is by itself inadequate to protect a cellular telephone system from misuse by counterfeiters.
While the cellular telephone is identified by its ESN and MIN, it cannot be considered as an authorized cellular telephone because it is not known whether the ESN and MIN have been transmitted by the authorized cellular telephone or a fraudulent cellular telephone. For purposes of the present description, a cellular telephone identified on the basis of the transmitted ESN and MIN is designated as an unauthenticated cellular telephone until it is determined to be an authorized cellular telephone or a fraudulent cellular telephone. The designation of the unauthenticated cellular telephone as authorized or fraudulent must be made on the basis of some form of analysis beyond a simple check of the ESN and MIN to see that these are valid numbers.
In an effort to provide additional security, some cellular systems and other wireless services, authenticate cellular telephones based on the radio frequency (RF) transmission of data by the cellular telephones during a call set-up process. Rather than identify the cellular telephone by its ESN and MIN alone, the system identifies the cellular telephone by its transmission characteristics. In this manner, the cellular system operator can reject calls from fraudulent cellular telephones even when those cellular telephones transmit valid ESN and MIN numbers. For example, in U.S. Pat. No. 5,005,210 issued to Ferrell on Apr. 2, 1991 ("the Ferrell patent"), a system is described that analyzes certain transmitter characteristics in an effort to identify the transmitter type. The system in the Ferrell patent analyzes the manner in which the modulator makes a transition to the designated carrier frequency. This transient response is used to identify the type of transmitter.
While the Ferrell patent describes one class of transmission characteristics that can be used as a fingerprint, other transmission characteristics are also known in the art. For example, U.S. Pat. No. 5,420,910 issued to Rudokas et al. on May 30, 1995 ("the Rudokas patent"), describes an identifier, such as a radio frequency signature, that can be used to positively identify an authorized cellular telephone or a fraudulent cellular telephone. Other types of signature authentication systems are also known in the art and need not be described herein. These transmission characteristics, from whatever source they are derived, can be processed in different manners to create a "fingerprint" of the individual transmitter. The analogy with fingerprints is used herein because each transmitter transmission characteristics are believed to be completely unique. The transmitter fingerprint can be used to determine whether the transmission characteristics of the unauthenticated transmitter match the stored fingerprint of the authorized transmitter corresponding to the transmitted ESN and MIN. In such manner, the fingerprint is used with cellular telephone calls to authenticate a cellular telephone.
While the transmitter fingerprint is believed to be unique, it is not static. The transmission characteristics can change over time as the transmitter ages or is exposed to other extreme environmental conditions, such as dropping the cellular telephone. Under these circumstances, the stored fingerprint may no longer provide an accurate measure of the transmission characteristics. The stored fingerprint must be periodically revised to compensate for changes in the transmission characteristics.
Therefore, it can be appreciated that there is a significant need for a system and method to permit the alteration of a transmitter fingerprint in a manner that assures satisfactory security of the cellular telephone system. The present invention provides this and other advantages as will be illustrated by the following description and accompanying figures.